Pressure swing adsorption (PSA) devices are used to separate at least one preferentially adsorbed component from at least one less readily adsorbed component in a feed fluid mixture. Gas separation by PSA is achieved by synchronized pressure cycling and gas flow reversals through a set of adsorber beds which adsorb the preferentially adsorbed component/s in the feed gas mixture. During each cycle, a pressurized feed gas mixture is first introduced to the feed end of the adsorber beds. The less readily adsorbed component/s pass through the adsorber beds while the preferentially adsorbed component/s are adsorbed. Thus, gas taken from the end opposite the feed end of the bed (i.e. the product end) is concentrated in the less readily adsorbed component/s. The adsorbent in the beds is regenerated later in the cycle by closing off the supply of pressurized feed gas mixture, reducing the pressure in the bed thereby desorbing the preferentially adsorbed component/s, and exhausting or purging them from the bed.
A simple PSA cycle can thus involve a single pressurization step in which gas concentrated in less readily adsorbed component/s is obtained from the product end of the beds, and a depressurization step in which gas concentrated in readily adsorbed component/s is exhausted from the feed end of the bed. However, to improve purity, yield, and efficiency, complex PSA cycles are typically employed in the art. These more complex cycles use de-pressurization and re-pressurization gas flows between feed and product ends of the adsorbent beds at various stages in the cycle. Multiple adsorption beds are required for these more complex PSA cycles.
Conventional commercial PSA devices currently employ fixed-bed adsorbents in the form of beads or pellets from about 1 mm to 4 mm in size. In order to achieve higher cycle speeds, the gas velocities within the adsorbent beds must increase, particularly for devices with multiple adsorbent beds. The maximum cycle speed for such conventional beaded bed PSA devices is however limited by such factors as bead fluidization, attrition, and also to some extent valve operation speeds and valve durability.
Rapid cycle PSA (RCPSA) devices have been recently developed that operate at cycle speeds greater than about 2 cycles per minute. The use of structured adsorbent beds comprising laminated sheets of immobilized adsorbent avoids issues of bead fluidization and attrition and also allows for decreased pressure drops in the beds. The use of such laminated sheet adsorbent, combined with the use of compact high speed rotary valves allows high PSA cycle speeds to be achieved at high efficiencies.
U.S. Pat. Nos. 4,968,329 and 5,082,473 and application number 2002-0170436 disclose preferred embodiments for a RCPSA bed comprising spirally wound adsorbent sheets of 1 mm or less in thickness. An adsorbent sheet is spirally wound together with a spacer sheet, e.g. a wire mesh spacer sheet, such that the spacer sheet defines flow channels between adjacent sheets of adsorbent. U.S. Pat. No. 5,082,473 suggests that the ratio of half sheet adsorbent thickness to channel gap (b/t) is desirably near unity but could be between 0.5 and 2.0, or in other words, the channel gap could be somewhere between 0.25 to 1 that of the adsorbent sheet thickness. This implies then that the channel fraction in the bed (where channel fraction is defined as the ratio of the channel volume to the total bed volume) is less than 50%.
In many PSA applications, the feed streams may contain small amounts of contaminants that are even more preferentially adsorbed on the adsorbent than those component/s intended to be adsorbed. Such contaminants may be characterized by very strong, and sometimes irreversible, adsorption and may deactivate or poison the adsorbent thereby degrading its capacity and selectivity and thus its ability to function properly. For instance, high nitrogen selectivity, cation exchanged, low silica-to-alumina ratio zeolites are commonly used in the separation of oxygen from air, but these zeolites are very sensitive to water contaminant in the feed stream.
Various methods may be used in conventional PSA to remove contaminants from the feed gas stream and thus guard against degradation of the adsorbent bed. These include upstream clean-up of the feed gas (e.g. feed gas cooling followed by condensation upstream of the PSA device) or adsorption onto regenerable guard beds (which are typically placed at the feed end within the same adsorbent housing of the PSA device). The guard beds serve to adsorb virtually all the contaminant from the feed stream before it reaches the primary adsorbent bed. And, the guard beds are regenerated at the same time as the primary adsorbent bed in the typical PSA cycle. For removal of water contaminant from a feed stream, a dessicant is typically used as a guard layer at the feed end of the beds.
Guard layers for contaminant control within a PSA bed do not contribute to the primary adsorption process and thus effectively add undesirable dead volume to the PSA bed. Preferably, the void space at the ends of the adsorbent beds should be minimized for better recovery. It is thus desirable to minimize the length and internal void volume of such guard layers, while still effectively removing the contaminants in the feed stream. In the prior art, this is generally done by maximizing the amount of guard adsorbent material present in the guard layer while still allowing for acceptable flow of gas through the guard layer. In PSA applications employing zeolite adsorbents in which water is a primary contaminant, typically from 5 to 30% of the adsorber bed is occupied by a guard layer containing alumina, silica gel, activated carbon, or a combination of these. The feed gas is dried to 0.1 to 5 ppm of water vapour before contacting the zeolite adsorbent layers.
Conventional PSA devices are less sensitive to the presence of contaminants in the feed stream than are the recently developed, faster cycle RCPSA devices. The former have relatively longer adsorber beds over which contaminant diffusion must occur and have relatively larger adsorbent inventory so that if a given amount is deactivated, it represents a smaller fraction of the total. Further, the rate of deterioration is dependant on the cumulative number of cycles experienced, which is less for conventional PSA devices over a given time period.
The unexpected sensitivity of RCPSA devices to feed stream contaminants was noted experimentally in U.S. Pat. No. 7,037,358. Various methods were also disclosed therein to protect RCPSA devices against contaminants and particularly against water. For instance, the use of guard layers, similar in design to the primary adsorbent layers, were employed at the feed end of the adsorber beds. The layers generally were disclosed as being thin and having a high surface area, with the flow channels having narrow hydraulic radius in order to overcome mass transfer constraints. It is necessary to reduce the length of narrow flow channels in order to maintain a desirable low pressure drop across the guard bed.